The present application relates generally to a fuel supply system on a turbomachine; and more particularly to, a method of operating a turbomachine having a fuel supply system with at least one unchoked valve.
Some turbomachines, such as, but not limiting of, a gas turbine, an aero-derivative turbine, or the like, have multiple fuel supply systems. These fuel supply systems generally feed into at least one combustion can of a combustion system. A primary fuel supply system may use a natural gas as the fuel source; and a secondary fuel supply system may use a synthetic gas (hereinafter “syngas”) as the fuel source. Each fuel supply system may operate as the sole fuel source to the combustion system. Alternatively, these fuel supply systems may simultaneously supply fuel to the turbomachine.
Turbomachines generally include a compressor, a combustion system having a plurality of combustion cans, a fuel supply system, and a turbine section. Typically, the fuel supply system delivers a fuel, such as, but not limiting of, methane to the combustion system. Some turbomachines include a Dry-Low Nox (DLN) controls system; which may require the use of a “choked” valve for providing a choked flow.
Generally, “choking flow” occurs when a fluid flows at a sonic velocity. When the sonic velocity is reached the pressure upstream of the valve should be increased in order to increase the flow downstream of the valve. The differential pressure (DP) across the valve typically creates the sonic flow.
Choked flow may provide system stability by not reacting to pressure fluctuations downstream of the valve, which may be the result of changing conditions in the turbomachine. Choked flow is typically beneficial to turbomachine operation because downstream pressure fluctuations may not create a flow disturbance in the fuel supply system.
For a few reasons, the costs associated with incorporating a choked valve into a fuel supply system are higher that an unchoked valve. A choked valve in a fuel supply system typically costs more than an unchoked valve. The pressure drop across a choked valve is higher than an unchoked valve. This generally requires that a gas compressor, or the like, perform more work to meet the necessary pressure requirement.
Non-DLN types of combustion systems generally do not require a choked valve. For example, but not limiting of, combustion systems using a syngas typically do not require choked flow. These systems may burn the syngas using a diffusion form of combustion. The pressure range upstream of an unchoked valve may be of a broader range than that of a choked valve. Here, an upstream gas compressor performs less work to maintain the pressure upstream of the unchoked valve. However, these systems may require a flow meter to control the fuel flow to the syngas fuel supply system. The flow meter adds cost and may introduce reliability concerns to the non-DLN combustion system.
For the aforementioned reasons, there is a need for a method of reducing the cost of operating a turbomachine comprising the primary and secondary combustion systems. The method should not require a flow meter to control a fuel supply system comprising at least one unchoked valve.